Radiation selective absorbing coating and process for obtaining the same at room temperature

ABSTRACT

The present invention relates to materials and its thermal applications in different uses. More specifically, it relates to the use of a selective absorbing coating which is obtained at room temperature and is deposited on metal, which is additionally used for capturing solar energy or artificial light and converting it into thermal energy. The novelty of the present invention is related to its production process and its use and industrial application in meshes, fabrics, threads, fibers or metallic wires used in the textile industry for the manufacture of jackets, trousers, scarves, shirts, hats, gloves, mittens and mitts, sleeping bags, tents, to provide properties to absorb sunlight or artificial radiation and convert it into heat for heating said fibers or a body.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.14/895,362 filed Dec. 2, 2015, entitled, RADIATION SELECTIVE ABSORBINGCOATING AND PROCESS FOR OBTAINING THE SAME AT ROOM TEMPERATURE, pending,the content of which is incorporated herein by reference, which is a USnational stage entry of PCT/MX2014/000173 filed Nov. 4, 2014, under theInternational Convention, which claims priority over Mexican patentapplication No. MX/a/001213 filed Jan. 29, 2014.

FIELD OF THE INVENTION

The instant invention relates to materials and their thermalapplications in different purposes. More specifically, it relates to theemployment of a selective absorbing coating obtained at room temperatureand deposited on metal, used for harnessing solar energy and convertingit into thermal energy. Said coating improves the efficiency in thecollection of thermal energy, optimizing visible and near infrared lightharnessing and minimizing heat emission of the metal towardsenvironment.

OBJECT OF THE INVENTION

The object of the instant invention is to present the process forobtaining a solar and/or artificial radiation selective absorbingcoating at room temperature which can operate in the temperature rangefrom 0° C. to 300° C. and its application on metallic substrates withdifferent shape and geometry configurations such as smooth, rough,porous, tubular or laminar, as non-limiting exemplary embodiments of theinstant application, generating heat through solar radiation and/orartificial illumination.

It also relates to different uses and applications that can be given toit, for example, such technology can be used to coat metal fibers usedin the manufacture of coats, jackets, sweaters, hats, gloves, fabric fortents, shoes, boots, etc. without limiting the scope of the presentapplication, specifically as an inner linning (interlining) which isconstructed in each of the clothing designs or applications that canintegrate it so that these garments have the capacity to absorb solarand/or artificial radiation, convert it into heat, keep the calorificenergy and transmit said energy to the human body.

BACKGROUND OF THE INVENTION

In the field of solar energy, the selective absorbing coatingsefficiently capture solar radiation in the spectral region of highintensity visible light and near infrared. Consequently, a selectivecoating will absorb and retain a substantial amount of solar radiation,while a non-selective surface, such as an ordinary black body, will losea high percentage of the energy absorbed by re-radiation.

Absorbers with black surfaces absorb 95% of incident solar radiation.The reflection loss is only 5%. However, black surfaces give off much ofthis energy in the form of thermal radiation and wasting 45% of theabsorbed energy. Thus, the total yield of the collectors with blackcoatings is less than 50%. For high solar absorption applications, theselective coating must be thermally stable around 400° C., ideally inthe air and have an absorbency greater than 0.95 and a thermal emittancebelow 0.15 at 400° C.

The object of the selective absorbing coatings is to increase theefficiency of solar collectors and are generally used in thermosolarapplications. Said coatings have a large power of absorption of solarenergy and low emissivity characteristics in order to reduce energylosses through thermal radiation in the remote infrared region. Whatevertheir application, the selective absorbing coatings play an essentialpart in increasing the efficiency of heat absorbing materials.

There are two magnitudes denominated absorbance (α) in the UV_VIS regionof the spectre (200-1000 nm) and emittance (ε) in the infrared region(1-15 μm) used for evaluating the efficiency of selective absorbingcoatings. The greater a and the smaller c, the higher is the efficacy ofthe coating.

The selective coatings for the efficient absorption of solar energy andits conversion into heat are characterized because they have areflectance spectrum that changes abruptly according to the wavelengthvalue. Thus, with wavelength values below certain value (about 2 μm,corresponding to the infrared region), the intensity of solar radiationis null or with a very low value (about 5%), while with wavelengthsgreater than this value the intensity reaches a very high value (greaterthan 90%) which corresponds to the infrared spectral region. Thisensures that the heat acquired by the metallic element is not lostthrough thermal radiation.

Several patents and patent applications related to solar selectivecoatings are known. Usually, the coatings are made of a metal,dielectric or ceramic material substrate, at least one reflectingmetallic layer and at least one anti-reflection layer and their directapplication is in absorbing pipes for parabolic-trough solar collectorsand in absorbing sheets for solar panels, such as those described inpatents ES2316321B2, ES2317796B2 and patent application WO2012172148A1.The main advantage is an absorbance greater than 95% and an emittancelower than 0.20 in the range from 400° C. to 550° C. However, theircompositions and methods for obtaining thereof are very complex and thuswould not be economically sound in industries such as: food, textile,among others, because of their high production costs and thus the highprice of the final product would be high for such markets.

Specifically, the inventions described in patents ES2317796B2 orES2316321B2, report very acceptable absorbance values but theiremittance values are not so favorable, leading to a selectivity ratio ofα/ε 400° C.=0.975/0.15 and α/ε 400° C.=0.975/0.08.

Patent ES2317796B2 patent discloses a selective coating for solarapplications with a reflective coating in the infrared region betweentwo aluminum oxide layers, which allows any material of the reflectivelayer to not diffused in the infrared region in the superimposedabsorption layer; causing it to have a high absorption capacity α>95.5%and a reduced emissivity with ε<9% at an operating temperature of 550°C. under vacuum for a period of time of 250 hours, reporting aselectivity ratio of α/ε=10.61; but at medium and low temperatures thereare no reported results, or absorption operation and implementationcapacity.

Meanwhile patent application WO2012172148A1 refers to a selectiveabsorbing coating to visible and infrared radiation comprising: (a) afirst non diffusing barrier layer (2); (b) an IR reflective metalliclayer (3) of at least one metallic element selected from a groupconsisting of Au, Ag, Al, Cu, Ti and Pt; (c) at least one second nondiffusing barrier layer (4) formed by oxidation of the layer (3); (d) anabsorbent structure in the UV-VIS comprising at least a first film (5)and a second film (6) of cermet, which itself comprises a metal fractionof a metal selected from Pt, Cr, Mo, W, Zr, Nb, Ta and Pd, or any alloythereof, and a ceramic comprising a free oxygen nitride constituted by ametallic oxide selected from aluminum, silicon and chromium; and (e) anantireflective dielectric layer in the UV-VIS region comprising anitride of at least one metal selected from silicon, aluminum andchromium. Another object of the invention is the method for obtainingsuch a coating and its use in solar thermal collectors.

The ES2316321 patent reports that the methods for obtaining a selectivecoating in which the different layers of the coating are deposited bytechniques of physical vapor deposition in vacuum (PVD, physical vapordeposition) such as are thermal evaporation, electron gun, ionicimplantation or “sputtering”, by chemical vapor deposition (CVD) orthrough electrolytic baths, being the sputtering technique preferred forthis purpose. It also has a refractive index of between 1.4 and 2.4 ofthe dielectric material layers of the absorbent multilayered structurecomprising metallic oxides and/or nitrides of metallic elements.

The U.S. Pat. No. 4,104,134 describes a process for obtaining analuminum absorber panel through a chemical bath in an aqueous solutionwith an alkaline cleaner from 5 to 10 minutes at a temperature of 60° C.and 80° C. to be then immersed in a brilliant solution from 5 and 10minutes at 82° C. and 93° C.

Patent application WO2002072918 suggests a process and method forstripping a metal piece after welding to increase its corrosionresistance, indicating that the novelty of the invention resides inusing an acid and an alkaline stripping agent for the process containingheavy metals and subsequently adding sodium hydroxide to the process fora chemical precipitation; afterwards, in the passivation process forcreating an anticorrosion protective layer to the metal, it is possibleto maintained a temperature above 35° C.

The EP Patent 0317838 defines a method of manufacturing an ultra-blackcoating; detailing that the process of preparation is carried outthrough a chemical bath in electrolytic solution of phosphorus andnickel alloy to form the coating based, which is immersed generally attemperatures of between 80 and 95° C. from 1 to 5 hours. For the blackfinish, it is required to soak it into a nitric acid solution between a20° C. to 100° C. temperature from 5 seconds to 5 minutes, depending onthe phosphorus content in the base, which indicates that usually at aconcentration of 1 to 1 of phosphorus to nitric acid at a temperature of50° C. the coating base blackens, the typical process temperature variesfrom 30 to 80° C. with a time from 5 seconds to 5 minutes, achieving avery stable coating with excellent mechanical strength, moistureresistance, and a spectrum reflectance from 0.1-0.4%, a wavelength widthfrom 380-1800 nm and a wavelength range of 0.1% or less.

Particularly, in each one of these patents a large number of selectivecoatings have been described that use cermets formed by some of thefollowing metals: Cu, Ni, Co, Pt, Cr, Mo, W, Al or Ag; and as ceramicmatrix, the following compounds: SiO, SiO₂, Al₂O₃, AlN or MgO. In orderto improve their efficacy, these cermets must be covered with a layer ofa material having very good transparent qualities such as the followingoxides: Cr₂O₃, MoO₃, WO_(x), H_(f)O_(x) or SiO₂, where said layer actsas anti-reflection layer. Additionally, the cermet must be deposited onthe metal acting as infrared mirror, which is usually achieved with Ag,Cu, Al, Au or Pt.

Unlike the state of art, the present invention introduces a method forobtaining a coating at room temperature, and the invention presented isnot comprised of multiple layers. Nevertheless, very good results areobtained in the absorbent and reflective properties of the material. Onthe other hand, in any patent application or patent granted in ourknowledge, it is proposed depositing a selective coating into fibers,threads, wires or metallic mesh with minimum thickness from 0.03 mm,which expands the applicability of the invention to other industriesmainly textiles for making clothing that use solar selective coatingsthat provide solar and artificial radiation absorbent qualities.

The following patents disclose the background in the use of metallicfibers where we have found the following methods, processes and fiberproducts that integrate solar selective coatings.

The document U.S. Pat. No. 8,187,984 B2, Temperature sensitiveintelligent textiles, relates to a textile fabric that includes a smoothsurface with one or more regions (layers) of material having a variablebehavior of thermal expansion or contraction, adjusting the insulationperformance of the textile fabric in response to ambient conditions, yetthe patent does not teach the use of radiation-absorbing coatings.

The document KR101386765, Electrically conductive fiber of graphene andmethod of production thereof, relates to a method for manufacturing agraphene electronic conductive fiber coated using a cotton thread ofmodified surface and the grapheneosin solution. The electronicconductive fiber manufactured by such method has very high conductivity,thereby being used for the intelligent electronic fiber (e-textile ande-fiber); but its application is limited to the transmission ofelectricity.

The document KR101373633, Method for manufacturing a conductive metalfiber, that has a higher elastic resistance, method of manufacturingproducts with the metal fiber composition with use of the same. Thedocument relates to a method for manufacturing a fiber of a complexconductive metal that can be applied to an intelligent textile, made bycombining the use of technologies such as electricity, computers, andelectronics technology. In order to increase the limit of elasticity ofthe fiber by collecting multiple pieces of a first twisted threadtwisted into a thread and a conductive fiber. The method formanufacturing the conductive complex fiber comprises the steps of: afirst process that manufactures the fiber by winding the thread with thecoated conductive fiber; a second process that makes the first twistedstranded thread; and a third process that produces fiber reinforcedhaving a higher creep strength for winding the thread onto the surfaceof multiple pieces of the first coated stranded thread. This patentreflects the intention to bring to market intelligent fibers thatintegrate advanced technology for making clothing.

Finally, the document WO 2010129923 A2 entitled Pattern for controllingheating in materials, relates to method and apparatus using an array ofheat elements coupled to a base material to maintain close body heat,while maintaining the desired transfer properties of the base material.In some embodiments, the material elements that manage or control theheat include elements that reflect or conduct heat; the mainly usedmaterials are aluminum as reflective and can be glued, sewn or ironed tothe clothes so that they can be addressed into the body of a wearer oraway from the same in the inside of the garment; its method ofpreparation involves the sputtering technique for the precipitation ofthe material onto the fabric; unlike the present invention, this patentmust be superimposed by the inner layer of the clothing to reflectinfrared from the body, and it does not have the ability to absorb heatas the present invention does.

Overall, the patents discussed, analyze and give detail in the method ofmaking intelligent textile fibers with different applications; howeverno one deepens on the ability to use a selective coating method thatwill provide to mesh fibers, threads, fibers, and metallic wire,capacity to absorb solar and artificial radiation and to generate heat.At the same time, these patents propose processes that cannot be done atroom temperature, which involves high costs processes. Finally, unlikeprior art documents, this invention discloses a method for deposing aselective coating into fibers or metallic meshes in thicknesses from0.03mm without affecting the substrate.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1: shows a cross sectional view of the radiation selectiveabsorbing coating of the present invention comprising a substrate ofmetallic material (1) and a radiation absorbing metallic layer (2),which to exemplify it is a cross section of a coated metallic pipe;

FIG. 2: shows a graphic of the reflectance value measured in a sample ofthe present invention;

FIGS. 3A and 3B: show values of temperature increase of the selectivecoating obtained at room temperature when the sample is illuminated witha radiation of 1000 watts/m2. By varying the treatment time in theprocess of obtaining the selective coating, various surface colors ofthe coating according to the graph (FIG. 3A) and colors of the coating(FIG. 3B) are obtained: blue (3), gray (4), black (5), blue-purple (6),green (7), gold (8), lilac (9). Based on the rate of temperature rise itwas determined that the blue-purple has the highest ability to absorbradiation;

FIG. 4. shows a Mesh, wire, thread or metallic fiber (10) and itsmesh-shaped cross section (11) with the layer of radiation selectiveabsorbent (12);

FIG. 5: shows a preferred application of the metallic fiber with thesolar and/or artificial radiation selective absorbing coating at roomtemperature of the present invention, in the conformation of a fabricconsisting of an exterior layer of any type (13), fiber or metallic meshwith the solar and/or artificial radiation selective absorbing coatingat room temperature of the present invention (14), and the inner liningor insulating of the garment of any type (15);

FIG. 6: shows the integration of the fiber, thread, mesh or steel wirewith solar selective coating at room temperature as an interlining injackets (16), using the system described in FIG. 5.

FIG. 7: shows the integration of the fiber, thread, mesh or steel wirewith selective absorbing coating for use in shoes (17), gloves (18) caps(19) and tents (20) using the system described in FIG. 5;

FIG. 8: shows a comparative plot of temperature increase using thefollowing configuration, the mesh, fiber, fabric, or metallic wire withthe selective absorbing coating in the middle in a sandwich-type ortandem configuration with a polyester-cotton fabric with a weight of 235g/m², and insulating lining (21) and one of a similar configurationwithout the fiber, mesh, thread or wire with the selective absorbingcoating of the present invention as interlining between thepolyester-cotton fabric and the insulating lining (22).

FIG. 9: shows an outside temperature graph of a test at 3° C. of twopieces: a composite with the system described in FIG. 5 of the presentinvention (23) and another with aluminum and/or copper reflective dottechnology in the interior lining to allow recycle the infrared fromhuman body (24), indicating the temperature increase that jackets allowthat integrate the fiber mesh, thread or wire with the presentinvention;

FIG. 10: shows a photo with an infrared camera of the system describedin FIG. 5 with different components: without using mesh, fiber, thread,or metallic wire with the selective absorbing coating (25), using themesh, fiber, thread, or metallic wire with the selective absorbingcoating (26) and using the mesh, fiber, thread or metallic wire withoutthe selective absorbing coating (27);

FIG. 11: shows a graph of the absorption spectrum of the presentinvention in the UV (28), visible (29) and infrared (30) range; and

FIG. 12: shows a graph of transmittance of UV rays on the selectiveabsorbing coating obtained at room temperature, representing the valuesof UV rays that the present invention passes upon contact with light.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to an absorbent and selective coating ofsolar and/or artificial and infrared radiation. In the presentinvention, the term radiation is to be understood broadly and withoutlimitation, as the total electromagnetic spectrum, including sunlight,ultraviolet, infrared and artificial lighting, also called selectivesurface, which refers to a material or coating exhibiting opticalselectivity; said coating material has optical properties whichextremely vary from one spectral region to another, and it ischaracterized by its production process at room temperature. In thepresent invention, the term ambient temperature should be understood asthe temperature from the 20° C. to 40° C., including standard roomtemperature of 25° C. Said coating is and may be permanently secured orfixed to a base material in a plurality of forms, such as, withoutlimiting the scope of the methods: by chemical solution, cathodicspraying (sputtering), thermal evaporation, electrochemical and spray orsol-gel.

This coating acts as a heat trap, as it absorbs solar and/or artificialradiation and transforms it into heat. It also has the ability toreflect infrared radiation generated by a body.

Said coating comprises a substrate (1 and 10) of metallic material, thatmay present, without limitation, certain dielectric or ceramiccharacteristics, and at least a metallic layer (2 and 12) providing lowemittance properties and because of its features it has various uses,for example as selective absorbent on metallic surfaces or substratesfor thermosolar applications, in the textile industry.

The substrate (1 and 10) of metallic material may have surfaces ofvarious shapes, geometry and texture configurations, including, withoutlimitation, smooth, rough, porous, tubular, sheet, wire, threads,filaments, meshes, spheres and this without limiting the type of basematerial that can be used, provided it complies with the basic featuresfor the material or selective absorbing coating adheres to and coat thesurface maintaining the physical and chemical properties and performance(12).

For the above mentioned uses and applications, the solar and/orartificial radiation selective absorbing coating operates within thetemperature range from 0° C. to 300° C., in the case of solar thermalapplications, generating a range in the ratio of selectivity fromα/ε=5.33 to α/ε=4.23 between such temperatures; enough for itsemployment in devices generating heat through solar radiation orartificial illumination.

The steps in the process of obtaining the proposed invention comprises(a) at least one cleaning stage, (b) at least one first stage ofimmersion and standing in aqueous solution, (c) at least one firstrinsing stage, (d) at least one second stage of immersion in aqueoussolution and (e) at least one second rinsing stage.

In said cleaning stage, the metallic substrate or surface to be coatedis cleaned with solvents that are selected from the group comprising,without limiting the scope of the invention, the following substances:

A mixture of silicates, phosphates, carbonates and sulfates, to removeimpurities such as dust and some greases;

Trichloroethylene for removing greases and oils that may be present onthe metallic surface;

Acetone, for removing inorganic greases and polymer coatings that aredifferent from oxides.

After the cleaning stage, the substrate to a stripping process in anaqueous solution of hydrofluoric acid is subjected in a concentrationrange of 0% to 5% plus nitric acid at a concentration ranging from 5% to15%.

After the cleaning stage, the substrate is subjected to a strippingprocess in a hydrofluoric acid aqueous solution at a concentrationranging from 0% to 5% plus nitric acid at a concentration ranging from5% to 15%. In a period of time from 8 to 16 minutes, the surface to becoated is allowed to stand immersed in the solution.

Then, the water rinsing stage is conducted (distilled water may beused).

After the cleaning stage and immersion in aqueous solution, in thesecond immersion stage, the pre-treated substrate is immersed in achromic acid aqueous solution at a concentration ranging from 200 g/L to300 g/L and sulfuric acid at a concentration ranging from 350 g/L to 450g/L, during 9 to 10.5 hours obtaining coatings 3 blue, 4 gray, 5 black,6 blue-purple according to FIGS. 3A and 3B with an absorption rate of 80to 89% of solar radiation, or more during a time from 13 hours up to 24hours obtaining coatings 7 green, 8 gold, 9 lilac according to FIGS. 3Aand 3B with an absorption rate of 75% to 80% of solar radiation, andwherein an optimum coating is obtained preferably within 9.5 and 10.5hours (3, 4, 5).

This coating is generated by applying the indicated ranges and with aroom temperature, between 20° C. and 40° C., and at a humidity rangingfrom 0% RH to 80% RH, preferably between 20-80% RH, because out of thisrange precipitation would be generated in the solution.

Once obtained the first coatings, chromic acid solution can be kept atroom temperature and used repeatedly for more coatings without affectingthe absorption properties of coatings to be developed afterwards, whichpresents big savings on the cost of the solution.

Finally, the substrate with the coating is withdrawn and is submitted toa rinsing stage that can be conducted with water or with an impurityremoving liquid.

Then, the metal substrate (1, 10) is coated with one single layer (2,12) of chromium oxide having simultaneously reflecting andanti-reflecting characteristics.

The process may employ a step (f) or additional polishing process, toimprove the coating, whereas sheets, tubes and spheres can be polished(2), however, if this additional step is not used, as in the case ofwires, threads, fabric or metallic fiber, this does not drasticallyreduced absorbance values.

The usage of acetone is not mandatory in this procedure, this componentensures the cleaning of the metallic substrate (1) but it does notaffect the efficacy values obtained.

The absorption level in the wavelength from 0.25 to 1.0 μm is in therange from 80 to 89%, the reflectance level in the wavelength from 2 to15 μm which is in the range from 15 to 21%.

The thickness of the obtained film of chromium oxide is 100 nm to 200nm.

Also and despite it is performed with hydrofluoric acid and nitric acidin the pretreatment and chromic acid to the pre-treated substrate togenerate the selective coating at room temperature, the process allowsits application on metallic substrate with thicknesses smaller than 0.03mm up to thicknesses greater than 1.2 mm keeping their absorbance andreflectance properties for the applications described (12, 16, 17, 18,19, 20).

A variation is that the present invention can be depose equally on reelsof fiber, filament or metallic wire before being woven into a fabricwoven with points or planes, without limiting the scope of the wovenfabric, and having the same properties.

The tests conducted on the selective absorbing coating with a typicaloptical test of reflectance generate a high reflectance spectrum resultsuch as the one shown in FIG. 2. The reflectance spectrum abruptlychanges according to the value of the wavelength (approximately 1 μm),the intensity of the reflected solar radiation has a very low value,whereas at wavelengths greater than 1 μm, the intensity of the reflectedradiation reaches a very high value. This ensures that the heatingacquire by the metal element is not lost by thermal radiation.

The tests performed in FIG. 8, with the following configuration, mesh,fiber, fabric or metallic with the selective absorbing coating in themiddle in a sandwich-type or tandem configuration with apolyester-cotton fabric with a weight of 235 g/m², and interiorinsulating lining (21) and one of a similar configuration without thefiber, mesh, thread or wire with the selective absorbing coating of thepresent invention as interlining between the polyester-cotton fabric andthe insulating lining (22), both exposed to a radiation of 1000 W/m²emitted by two 500 W halogen lamps each at room temperature of 24° C.with an infrared filter, reflect a differential of 21.2° C. in just 5minutes of exposure to radiation, being better and superior the garmentthat includes fiber, mesh, thread, wire with the present invention (21)demonstrating the advantages of integrating the meshes, fibers, threador metallic wire with the present invention to traditional fibers toimprove the thermal and energy absorption properties.

According to FIG. 9, where the external temperature of two pieces ofgarment is exposed, the one which includes fiber, thread, mesh or wirewith the present invention (23) and one which includes the reflectivedot technology of the infrared from human body (24), both exposed toradiation of 1000 W/m² emitted by two 500 W halogen lamps each at roomtemperature of 3° C., demonstrating a temperature differential of up to5° C. in a time of 5 minutes, demonstrating that the present inventionhas advantages in the heat absorption and thermal storage, thatcomparatively with the traditional jackets among others, do not offer.

As it can be observed, the instant invention has the advantage of beinga simple process that however has not been previously used for solvingsituations of cost reduction implemented in industries where processheat is required in the manufacturing process and where fossil fuels aremainly used, and it is thus considered a novelty for its simplicity butwith a technical degree of good results.

Another advantage is that solvents and solutions can be reused, thusoptimizing the use of these supplies.

Also in FIG. 10 we note that the present invention (26) substantiallyimproves the uptake of radiation from traditional clothing (25),including those which are added only a fiber, mesh, thread or metallicwire beneath the outer fabric without the present invention (27). InFIG. 10, where we observe a thermal photo at different sandwich-typecompositions similar to FIG. 5, the temperature differential (26) is 22°C. higher than (25) which does not have any fiber, thread, mesh ormetallic wire and 10° C. higher than the one having only a fiber,thread, mesh or metallic wire but without the selective absorbingcoating (27), so we see that the novelty of the present invention alsoresides in the fact that the temperature of the garments and traditionaltextile fibers increases and therefore improve comfort when using thepresent invention. According to the values reported in FIG. 11, thepresent invention has the ability to absorb light in the range of thewavelength spectrum up to 60% of the ultraviolet region (28), up to 73%in the visible spectrum (29), and up to 89% in the infrared spectrum(30). This means that the present invention absorbs in these threeintervals of the electromagnetic spectrum: UV, visible, IR and turns itinto heat for use in the human body.

A big advantage is that the present invention integrated on mesh,thread, wire or metallic fabric functions as a protective shield againstultraviolet rays as the tests conducted to measure the transmittanceindicate that it only allows 27.6% of ultraviolet light incident on thesame as shown in FIG. 12, thus improving protection against ultravioletrays of jackets (16), caps (19), gloves (20), shoes (17), trousers,coats, tents (20), increasing a further 73% to the protection of thefabrics to be used outdoors and exposed to ultraviolet rays.

Another advantage is that it can be applied into substrates withthicknesses below 0.03 mm (10, 11, 12) and can be applied toconfigurations where selective coatings have never been applied as inthe case of fibers, wire, threads or metallic meshes for use in textileswherein jackets (16), trousers, scarves, shirts, hats (19), shoes (17)gloves (18), mittens and mitts, sleeping bags, tents (20), withoutlimiting the scope of the invention, can be made up, and that inconjunction with insulating textiles enhance absorption and retention ofbody heat and solar radiation.

Thus, one of the main uses of the selective coating obtained at roomtemperature is on fibers, threads, wire and/or metallic meshes (10) andan example of its direct industrial application is for making jackets(16), sweaters, hats (19), gloves (18), fabric for tents (20), shoes(17) boots (17) among others without limiting the scope of theinvention. Unlike the application on other substrates for other uses itdoes not require additional procedures in order to integrate theinvention into fabrics with different characteristics, qualities andcompositions as an interlining (14); the union of these absorb solarradiation, convert it into heat and maintain heat (26), which isequivalent to the use of bulky garments without the need of havingthermal insulation around the garment to protect from the cold. The mainadvantages of a fabric of the present invention are: lighter textilesand garments, less bulky, externally generated heat and retentionthereof in the garment, convenience, added value for the producer oftextiles and superior aesthetics.

What is claimed is:
 1. A radiation selective absorbing coating obtainedat room temperature comprising: a metallic substrate; and at least onemetal layer of chromium oxide; wherein said metal layer has an opticalabsorption selectivity in the electromagnetic spectrum, reflective andantireflective characteristics and absorbance values between 80%-89% andreflectance between 15%-21%.
 2. The radiation selective absorbingcoating according to claim 1, wherein the metal layer of chromium oxidelayer is 100 nm to 300 nm.
 3. The radiation selective absorbing coatingaccording to claim 1, wherein the optical absorption selectivity in theelectromagnetic spectrum is up to 60% in the ultraviolet region, up to73% in the visible spectrum and up to 89% in the infrared spectrum.
 4. Aprocess for obtaining at room temperature a radiation selectiveabsorbing coating of claim 1, comprising the steps of: (a) at least onecleaning stage; (b) at least one first stage of immersion and standingin aqueous solution of a metallic substrate to be coated; (c) at leastone first rinsing stage; (d) at least one second stage of immersion inaqueous solution; and (e) at least one second rinsing stage, (f)optionally, a polishing step, and wherein the above steps (a)-(e) areperformed between 20° C. and 40° C., and between 0% and 80% relativehumidity.
 5. The process according to claim 4, wherein in the at leastone cleaning stage (a), the metallic substrate to be coated is cleanedwith solvents that remove dirt, dust, grease, inorganic fats, polymericcoatings, coatings different from oxides and oils.
 6. The processaccording to claim 5, wherein the solvent is selected from the groupcomprising silicates, phosphates, carbonates, sulfates,trichloroethylene, and/or optionally acetone.
 7. The process accordingto claim 4, wherein in the at least one first stage of immersion andstanding in aqueous solution (b), the metallic substrate to be coated isimmersed in an aqueous solution of hydrofluoric acid in a concentrationrange from 0% to 5%, plus nitric acid in a concentration of 5% to 15%.8. The process according to claim 7, wherein the standing in aqueoussolution of a metallic substrate to be coated is for a time between 8and 16 minutes.
 9. The process according to claim 7, wherein themetallic substrate to be coated has a minimum thickness from 0.03 mm upto thicknesses greater than 1.2 mm.
 10. The process according to claim4, wherein the at least a first rinsing step (c) is performed withwater.
 11. The process for according to claim 10, wherein the rinsewater is distilled water.
 12. The process according to claim 4, whereinthe at least one second stage of immersion (d), the pretreated metalsubstrate is immersed for 9 to 24 hours in an aqueous solution ofchromic acid and sulfuric acid.
 13. The process according to claim 12,wherein the chromic acid and sulfuric acid have a concentration of 200g/L to 300 g/L and 350 g/L to 450 g/L respectively.
 14. The processaccording to claim 12, wherein the pre-treated metallic substrate isimmersed from 13 to 24 hours.
 15. The process according to claim 12,wherein the pre-treated metallic substrate is preferably immersed from9.5 to 10.5 hours.
 16. The process according to claim 4, wherein the atleast one second rinsing stage (e) is performed with water or with aliquid that removes impurities.
 17. The process according to claim 4,wherein a humidity is between 20-80% RH.
 18. A radiation selectiveabsorbing coating obtained from the process claim
 4. 19. The radiationselective absorbing coating according to claim 1, wherein the coating isdeposited on metallic substrates with different configurations, shapeand geometry.
 20. The radiation selective absorbing coating according toclaim 19, wherein the metallic substrate is selected from a metallicfiber, mesh, wire, fabric or steel wire for making interlining forcoats, jackets, sweaters, hats, gloves, tents, shoes, boots.
 21. Theradiation selective absorbing coating according to claim 19, wherein theconfiguration is tandem type to capture solar radiation and/orartificial light and convert it into heat to increase the temperature ofthe garment and the body.
 22. The radiation selective absorbing coatingaccording to claim 19, wherein the shape and geometry can be smooth,rough, porous, tubular or laminar.
 23. The radiation selective absorbingcoating according to claim 1, wherein the coating increases protectionof jackets, trousers, scarves, shirts, hats, gloves, mittens and mitts,sleeping bags, tents, against UV with 73% additional protection for eachapplication.
 24. The radiation selective absorbing coating according toclaim 1, wherein the coating is apply to clothing, tents, shoes, orboots to capture solar radiation and/or artificial light, convert itinto heat, retain the calorific energy and transmit it to the humanbody.
 25. The radiation selective absorbing coating according to claim24, wherein the radiation selective absorbing coating obtained at roomtemperature further reflects infrared from human body.
 26. The radiationselective absorbing coating according to claim 1, wherein the coating isapply to thermosolar applications in the textile industry.